Currently, commercial photoactivated dental restorative resins are based on dimethacrylates and the reaction mechanism is through chain-growth free radical polymerization. Existing dimethacrylate systems are popular for fillings and other dental prostheses because of their aesthetic merit and “cure-on-command” feature. Acrylic monomers currently in use generally comprise linear aliphatic or partially aromatic core groups with terminal methacrylate functionality such as 2,2-bis[p-(2′-hydroxy-3′-methacryloxypropoxy)phenyl]-propane (commonly referred to as BisGMA), urethane dimethacrylate UDMA (UDMA; product of 2,2,4(2,4,4)-trimethyl-hexylisocyanate and hydroxyethyl methacrylate) or polyurethane dimethacrylate (PUDMA). As Bis-GMA, PUDMA and other resins are highly viscous at room temperature, they are generally diluted with an acrylate or methacrylate monomer having a lower viscosity, such as trimethylol propyl trimethacrylate, 1,6-hexanediol dimethacrylate, 1,3-butanediol dimethacrylate, and the like. Other diluents include dimethacrylate monomers, such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate (PEGDMA) and tetraethylene glycol dimethacrylate. A photoinitiator may be included to provide photoactivity. Dental restorative compositions also typically include silanized filler compounds such as barium, strontium, zirconia silicate and/or amorphous silica to match the color and opacity to a particular use or tooth.
The photoactivated restorative materials are often sold in separate syringes or single-dose capsules of different shades. If provided in a syringe, the user dispenses (by pressing a plunger or turning a screw adapted plunger on the syringe) the necessary amount of restorative material from the syringe onto a suitable mixing surface. The material is then placed directly into the cavity, mold or location of use. If provided as a single-dose capsule, the capsule is placed into a dispensing device that can dispense the material directly into the cavity, mold, etc. After the restorative material is placed, it is photopolymerized, or cured, by exposing the dental restorative composition to the appropriate light source. The resulting cured polymer may then be finished or polished as necessary with appropriate tools. Such dental restoratives can be used for direct anterior and posterior restorations, core build-ups, splinting and indirect restorations including inlays, onlays and veneers.
However, conventional acrylic monomers and indeed their polymers have several critical deficiencies that limit their clinical performance in dental restorative compositions. Existing dimethacrylate monomers and materials demonstrate relatively low conversion, excessive polymerization shrinkage, poor toughness and excessive water uptake. The current systems can only reach a final double bond conversion of 55 to 75 percent, which not only contributes to insufficient wear resistance and mechanical properties, but also jeopardizes the biocompatibility of the composites due to the leachable unreacted monomers. Dimethacrylate based resins exhibit significant volumetric shrinkage during polymerization and the induced shrinkage stress results in tooth-composite adhesive failure, initiating microleakage and recurrent caries, which significantly reduces the longevity and utility of the dental restorative composite. Furthermore, as one tries to increase the final double bond conversion to reduce the unreacted monomers, volumetric shrinkage and shrinkage stress unfortunately also increases, which has been a persisting problem since the development of this class of resins.
Generally, there is a need for dental restorative compositions that are sufficiently viscous to allow easy use but that upon polymerization exhibit lower shrinkage but higher conversion than existing dimethacrylate systems.